They have a robust paying subscriber base that supports them and don't have an owner whose last name rhymes with Pesos who can axe a story just because he doesn't like what it says.
That they publish articles that put Republicans in an unfavorable light is I think because Republicans are doing things that put themselves in an unfavorable light.
To your point, there have been at least a few articles I've seen that put Democrats in an unfavorable light as well.
And for what it's worth I consume news outlets that lean both ways. What's more important to me is factual accuracy.